1. Field of the Invention
The present invention is directed to the field of spark generating devices in general. More particularly, the present invention is directed to that portion of the above-noted field which is concerned with the generation of an electrical spark for purposes of ignition of an air/fuel mixture. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with devices for generating a spark for igniting the air/fuel mixture of an automotive internal combustion engine. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the generation of electrical sparks capable of igniting air/fuel mixtures having excess quantities of air or which have been diluted by the inclusion of combustion by-products through recirculation of exhaust gases (hereinafter collectively referred to as "lean charges"). More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with multiple air gap spark generation for the ignition of a lean charge.
2. Description of the Prior Art
The above-noted copending commonly assigned patent applications describe spark plug devices having multiple air gaps formed by arranging the high voltage and ground electrode of the spark plug device (hereinafter referred to as the primary electrodes) at a very great spatial separation and by interposing one or more intermediate or secondary electrodes in the air gap so formed to define a plurality of linearly arranged air gaps. These intermediate electrodes are coupled by high impedance means, such as for example capacitive coupling or resistive coupling, to a source of voltage which is electrically stable, at least for the time period during which the spark generation occurs. This arrangement has been observed to cause the multiple air gaps so formed to be sequentially broken down and, once broken down, to remain broken down until all of the applied electrical energy has been dissipated. These devices have been shown to achieve their principle objectives in generating an effective spark length which may be made to be several multiples of the spark length which may be generated in a single air gap with a particular applied voltage differential. These spark plugs have shown themselves to be capable of reliably igniting lean charges. However, in the use of these spark plugs it has been observed that each of the individual air gaps appears to be sustaining an individual spark.
The technical literature directed to analysis of the propagation of a flame front in a lean charge reports that an "ignition delay" occurs between the application of energy to generate and sustain an electrical spark and generation of the flame front which will cause combustion of the lean charge. Thus, in the spark plug devices according to the above-noted copending applications, the generation of a plurality of individual sparks is accompanied by a plurality of "ignition delays" in achieving ignition of the lean charge. In order to prevent the occurrence of multiple "ignition delays" while otherwise achieving the objectives of the above-noted inventions, it is an object of the present invention to provide a multiple air gap spark plug device capable of generating a single spark kernel. It is also an object of the present invention to provide an improvement in the spark plug devices according to the copending commonly assigned patent applications identified above by providing a spark plug device having multiple air gap forming secondary electrodes which may sequentially generate a plurality of sparks which may thereafter coalesce into a single spark kernel.
Such an individual spark kernel is capable of producing higher internal temperatures which will aid in the ignition of lean charges. It is therefore a further and specific object of the present invention to provide a multiple air gap spark generating device having a plurality of spark electrodes arranged for generation of a single, hotter, spark kernel. In the accomplishment of the foregoing objective, it is an object of the present invention to provide a multiple air gap spark plug device having a pair of high voltage or primary electrodes with one or more secondary electrodes wherein the primary electrodes are arranged to define a spark gap and each secondary electrode is positioned to have a sparking surface which is proximate to, but which does not obstruct, the spark gap defined by the primary electrodes. It is also an object of the present invention to provide such a device which may be conveniently and readily manufactured using present production facilities.